The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.
A quantum key distribution system transmits a single photon to a recipient after loading key information by adjusting polarization or phase of the single photon. The recipient extracts the key information using a polarization receiver, an optical phase modulator, and so on. This single-photon transmission is achieved using optical communication technologies, and the quantum key distribution system targeting long-distance transmission may primarily use a single mode optical fiber as a quantum channel. When a polarization-modulated single photon is transmitted through the single-mode optical fiber, polarization properties become unstable, thus deteriorating transmission performance. Therefore, a phase modulation scheme is preferred to a polarization modulation scheme for the key distribution.
A phase modulation-based quantum key distribution system mainly uses a time-division optical interference scheme. The time-division optical interference scheme can be induced by using an asymmetric optical interferometer, an optical phase modulator, and so on. An asymmetric optical interferometer is configured to have two optical paths with different length for generating optical interference. Single photons inputted to the asymmetric optical interferometer are split into two probability distributions with different coordinates in the time domain. The optical phase modulator modulates the phase of the single photons passing through one of these paths. A receiving-end asymmetric optical interferometer divides the probability distributions into four coordinates in the time domain. If the asymmetric optical interferometers of the transmitter and the receiver have the same path difference, interference occurs between two neighbors among the four probability distributions of the single photon due to their overlap. The receiver also includes an optical phase modulator that modulates the phase of a single photon. The inventor(s) has noted that when the sum of the phase modulations from the transmitting and receiving ends is 2nπ, where n is an integer, the two superpositioned probability distributions of the single photons interfere constructively to exhibit a maximal detection probability. The inventor(s) has noted that in contrast, when the sum is (2n+1)π, the two superimposed probability distributions of the single photons interfere destructively, resulting in the minimal detection probability. The inventor(s) has noted that this implies that the performance of the optical interferometer affects the overall performance of a quantum cryptography system.
The inventor(s) has experienced that to obtain excellent optical interference, it is important to secure stable polarization and phase properties of the optical interferometer. The inventor(s) has noted that Two single photons interfering with each other in the receiver need to have equal polarization, and the phase needs to be kept constant throughout the optical path except for the total phase modulation amount additionally provided by the optical phase modulators. The inventor(s) has noted that for this purpose, optical interferometers have precise configurations, and a phase compensated control could be optionally performed as a complementary measure.
To address the instability of known interferometers arising from positioning an optical phase modulator on the interferometer path, Korean Patent Application Publication No. 10-2011-0071803 has proposed, as illustrated in FIG. 1, positioning the optical phase modulator external to the interferometer in order to simplify the configuration of the optical interferometer and to mitigate difficulties, such as an extension of an optical path caused by the optical phase modulator, enhanced instability and insertion loss in the optical interferometer.